Population Dynamics Of Diatoms Research Articles

Agent-Based Model of Diatom Population Dynamics in Marginal Arctic Seas in Summer

Agent-Based Model of Diatom Population Dynamics in Marginal Arctic Seas in Summer

Agent-Based Model of Diatom Population Dynamics in Marginal Arctic Seas in Summer

A hypothesis is proposed for the functioning of the diatom population under conditions of water column stratification. It is assumed that cells in the upper illuminated water layer accumulate biomass due to photosynthesis. In the lower layer, rich in minerals, the cells replenish their intracellular reserves. Moving from one layer to another allows to cells to compensate for the lack of resources. Sinking occurs as a result of sedimentation, and lifting occurs due to the release of carbon dioxide microbubbles in the process of respiration. On the basis of this hypothesis, an agent model of the population is constructed. The calculations have shown that vertical movements ensure the existence of the population. Various surface illumination make it possible to obtain various distributions of cell numbers in the upper and lower water layers, which is consistent with the data of expeditionary observations.

Modern diatom assemblages and their association with meteorological conditions in two lakes in northeastern Poland

The population dynamics of diatoms are affected by a variety of environmental variables. Due to their short generation times and high sensitivity to changes in physicochemical conditions, diatoms are considered good environmental indicators. The main goal of our study was to find and explain the relationships between changes in meteorological conditions and diatom fluxes and taxonomic composition based on the example of two small lakes: Łazduny and Rzęśniki. Using meteorological data, sediment traps, and regular measurements of limnological and hydrochemical properties of the water column, we collected a three-year-long, high-resolution series of observations. The results show that total diatom fluxes are indirectly influenced by changes in meteorological conditions, acting through changes in the mixing regimes that determine the nutrient and light availability in lakes. Statistical analyses showed that the variability of the diatom data is correlated with air temperature and wind speed. Nevertheless, their influence on diatom assemblages is most likely the surrogate for the complex changes in the physical structure of the investigated lakes. Despite many similarities between the studied lakes such as mixing regime patterns, dominant diatom taxa, and seasonal dynamics of diatom fluxes, we recorded differences in both the seasonal succession of specific diatom taxa and the occurrence of the peaks of total fluxes, and differences in taxonomic composition referring to less dominant taxa. We attribute these dissimilarities to the local conditions, such as the hydrological types of the lakes, the extent of the littoral zone, and exposure to the sunlight connected to the position in the catchment.

Is the diatom sex clock a clock?

The unique life cycle of diatoms with continuous decreasing and restoration of the cell size leads to periodic fluctuations in cell size distribution and has been regarded as a multi-annual clock. To understand the long-term behaviour of a population analytically, generic mathematical models are investigated algebraically and numerically for their capability to describe periodic oscillations. Whereas the generally accepted simple concepts for the proliferation dynamics do not sustain oscillating behaviour owing to broadening of the size distribution, simulations show that a proposed limited lifetime of a newly synthesized cell wall slows down the relaxation towards a time-invariant equilibrium state to the order of a hundred thousand generations. In combination with seasonal perturbation events, the proliferation scheme with limited lifetime is able to explain long-lasting rhythms that are characteristic for diatom population dynamics. The life cycle thus resembles a pendulum clock that has to be wound up from time to time by seasonal perturbations rather than an oscillator represented by a limit cycle.

A 2-decade (1988–2009) record of diatom fluxes in the Mauritanian coastal upwelling: impact of low-frequency forcing and a two-step shift in the species composition

Abstract. Eastern boundary upwelling ecosystems (EBUEs) are among the most productive marine regions in the world's oceans. Understanding the degree of interannual to decadal variability in the Mauritania upwelling system is crucial for the prediction of future changes of primary productivity and carbon sequestration in the Canary Current EBUE as well as in similar environments. A multiyear sediment trap experiment was conducted at the mooring site CBmeso (“Cape Blanc mesotrophic”, ca. 20∘ N, ca. 20∘40′ W) in the highly productive coastal waters off Mauritania. Here, we present results on fluxes of diatoms and the species-specific composition of the assemblage for the time interval between March 1988 and June 2009. The temporal dynamics of diatom populations allows the proposal of three main intervals: (i) early 1988–late 1996, (ii) 1997–1999, and (iii) early 2002–mid 2009. The Atlantic Multidecadal Oscillation (AMO) appears to be an important driver of the long-term dynamics of diatom population. The long-term AMO-driven trend is interrupted by the occurrence of the strong 1997 El Niño–Southern Oscillation (ENSO). The extraordinary shift in the relative abundance of benthic diatoms in May 2002 suggests the strengthening of offshore advective transport within the uppermost layer of filament waters and in the subsurface and in deeper and bottom-near layers. It is hypothesized that the dominance of benthic diatoms was the response of the diatom community to the intensification of the slope and shelf poleward undercurrents. This dominance followed the intensification of the warm phase of AMO and the associated changes of the Atlantic Meridional Overturning Circulation. Transported valves (siliceous remains) from shallow Mauritanian coastal waters into the bathypelagic should be considered for the calculation and model experiments of bathy- and pelagic nutrients budgets (especially Si), the burial of diatoms, and the paleoenvironmental signal preserved in downcore sediments. Additionally, our 1988–2009 data set contributes to the characterization of the impact of low-frequency climate forcings in the northeastern Atlantic and will be especially helpful for establishing the scientific basis for forecasting and modeling future states of the Canary Current EBUE and its decadal changes.

Asparagine-based production of hydrogen peroxide triggers cell death in the diatom Phaeodactylum tricornutum

ABSTRACT As a major component of coastal phytoplankton blooms, diatoms are an important source of dissolved organic matter (DOM) in the ocean. While this DOM pool fuels the growth of diverse bacterial populations, diatom population dynamics itself is considered unaffected by it. Here, we identified strong negative effects of the amino acid asparagine on the growth of the marine model diatom Phaeodactylum tricornutum. Using microscopic analyses and physiological assays, we showed that while asparagine fuelled photosynthetic growth, its supplementation to NO3 − medium significantly decreased the culture’s cell density and photosynthetic capacity (Fv/Fm) during the stationary phase of the growth cycle. We further showed that these effects resulted from asparagine-based production of hydrogen peroxide (H2O2) that triggered a massive cell death event. Effective lethal asparagine concentrations were as low as 12.5 µM, while 1 μM affected Fv/Fm, but did not trigger culture collapse. Based on sequence similarity with the characterized H2O2-producing L-amino acid oxidase (LAAO) of Chlamydomonas reinhardtii, LAO1, we identified putative LAO1 orthologs in the genomes of P. tricornutum and the toxigenic pennate species Pseudo-nitzschia multiseries. The molecular phylogeny of this gene indicates a shared ancestry with other algal LAAOs and suggests a conserved oxidative deaminase function that could mediate DOM release from diatom populations through the triggering of autocatalytic cell death events.

Flux variability of phyto- and zooplankton communities in the Mauritanian coastal upwelling between 2003 and 2008

Abstract. Continuous multiyear records of sediment-trap-gained microorganism fluxes are scarce. Such studies are important to identify and to understand the main forcings behind seasonal and multiannual evolution of microorganism flux dynamics. Here, we assess the long-term flux variations and population dynamics of diatoms, coccolithophores, calcareous and organic dinoflagellate cysts, foraminifera and pteropods in the eastern boundary upwelling ecosystem of the Canary Current. A multiannual, continuous sediment trap experiment was conducted at the mooring site CBeu (Cap Blanc eutrophic; ∼20∘ N, 18∘ W; trap depth is ca. 1300 m) off Mauritania (northwest Africa), between June 2003 and March 2008. Throughout the study, the reasonably consistent good match of fluxes of microorganisms and bulk mass reflects the seasonal occurrence of the main upwelling season and relaxation and the contribution of microorganisms to mass flux off Mauritania. A clear successional pattern of microorganisms, i.e., primary producers followed by secondary producers, is not observed. High fluxes of diatoms, coccolithophores, organic dinoflagellate cysts, and planktonic foraminifera occur simultaneously. Peaks of calcareous dinoflagellate cysts and pteropods mostly occurred during intervals of upwelling relaxation. A striking feature of the temporal variability of population occurrences is the persistent pattern of seasonal groups contributions. Species of planktonic foraminifera, diatoms, and organic dinoflagellate cysts typical of coastal upwelling, as well as cooler-water planktonic foraminifera and the coccolithophore Gephyrocapsa oceanica, are abundant at times of intense upwelling (late winter through early summer). Planktonic foraminifera and calcareous dinoflagellate cysts are dominant in warm pelagic surface waters, and all pteropod taxa are more abundant in fall and winter when the water column stratifies. Similarly, coccolithophores of the upper and lower photic zones, together with Emiliania huxleyi, and organic dinoflagellate cysts dominate the assemblage during phases of upwelling relaxation and deeper layer mixing. A significant shift in the “regular” seasonal pattern of taxa relative contribution is observed between 2004 and 2006. Benthic diatoms strongly increased after fall 2005 and dominated the diatom assemblage during the main upwelling season. Additional evidence for a change in population dynamics is the short dominance of the coccolithophore Umbilicosphaera annulus, the occurrence of the pteropod Limacina bulimoides and the strong increase in the flux of calcareous dinoflagellate cysts, abundant in warm tropical oligotrophic waters south of the study area after fall 2005. Altogether, this suggests that pulses of southern waters were transported to the sampling site via the northward Mauritania Current. Our multiannual trap experiment provides a unique opportunity to characterize temporal patterns of variability that can be extrapolated to other eastern boundary upwelling ecosystems (EBUEs), which are experiencing or might experience similar future changes in their plankton community.

Population Dynamics of Diatoms in a Spring-Neap Tidal Cycle in an Estuarine Creek of the Bonny River System

Population Dynamics of Diatoms in a Spring-Neap Tidal Cycle in an Estuarine Creek of the Bonny River System

Expression of death-related genes and reactive oxygen species production in Skeletonema tropicum upon exposure to the polyunsaturated aldehyde octadienal

<p>The effects of 4<em>E</em>/<em>Z</em>-octadienal (OCTA) on <em>ScDSP-1 </em>and <em>ScDSP-2 </em>gene expression and reactive oxygen species (ROS) production were investigated in the marine diatom <em>Skeletonema tropicum</em> (formerly <em>costatum</em>) using qRTPCR and flow cytometry. <em>ScDSP-1 </em>and <em>ScDSP-2 </em>genes have been previously shown to be involved in cell death in ageing cells and in response to photosynthetic stress. OCTA induced a differential, concentration-dependent <em>DSP</em> gene expression associated to ROS production, 821.6 and 97.7 folds higher for <em>ScDSP-1</em> and <em>ScDSP-2</em>, respectively. Among the concentrations tested, only 8 μM OCTA, which caused a reduction of 50% in cell concentrations at 24 h, was able to elicit an expression pattern consistent with a signalling role. Interestingly, only intermediate levels of reactive oxygen species (ROS) (<em>i.e</em>., 1.5±0.1 increase) were observed to be elicited by such concentration. These results suggest that ROS are key components of the molecular cascade triggered by polyunsaturated aldehydes (PUA) and leading to cell death. This could have implications for bloom final stages at sea, where PUA may act as effectors of diatom population dynamics through ROS acting as modulators.</p>

Discovery of two novel viruses expands the diversity of single-stranded DNA and single-stranded RNA viruses infecting a cosmopolitan marine diatom.

Recent studies have suggested that diatom viruses are an important factor affecting diatom population dynamics, which in turn are important in considering marine primary productivity. The marine planktonic diatom Chaetoceros tenuissimus Meunier is a cosmopolitan species and often causes blooms off the western coast of Japan. To date, two viruses, C. tenuissimus DNA virus (CtenDNAV) type I and CtenRNAV type I, have been identified that potentially affect C. tenuissimus population dynamics in the natural environment. In this study, we successfully isolated and characterized two additional novel viruses (CtenDNAV type II and CtenRNAV type II). This paper reports the basic characteristics of these new viruses isolated from surface water or sediment from the Hiroshima Bay, Japan. The physiological and morphological characteristics of the two new viruses were similar to those of the previously isolated viruses. However, the amino acid sequences of the structural proteins of CtenDNAV type II and CtenRNAV type II were clearly distinct from those of both type I viruses, with identity scores of 38.3% and 27.6%, respectively. Our results suggest that at least four genetically distinct viruses sharing the same diatom host are present in western Japan and affect the population dynamics of C. tenuissimus. Moreover, the result that CtenRNAV type II lysed multiple diatom species indicates that RNA viruses may affect various diatom populations in the natural environment.

Distribution of viable diatom resting stage cells in bottom sediments of the eastern Bering Sea shelf

Information on diatom resting stages is fundamentally important to understanding the population dynamics of diatoms including bloom formation. The distribution of viable diatom resting stage cells in bottom sediments of the eastern Bering Sea in July 2009 was investigated by the most probable number (MPN) method. The abundances of diatom resting stage cells ranged from 1.7×103 to 1.2×106MPNcellscm−3 wet sediment, comparable to those in shallow eutrophic areas where diatom blooms frequently occur. Common species during the spring phytoplankton bloom in the eastern Bering Sea were also dominant in sediments as resting stage cells. It should be noted that relatively high numbers of ice algae species, especially ribbon-shaped chain forming pennate diatoms, were found in the sediments. The life cycle strategy using resting stage cells allows planktonic and ice algal species to survive unfavorable environmental conditions such as the dark winter season, and potentially contribute to form blooms of several types (subsurface of ice, ice edge, plankton) through vertical mixing.

PLASTICITY OF OXYLIPIN METABOLISM AMONG CLONES OF THE MARINE DIATOM SKELETONEMA MARINOI (BACILLARIOPHYCEAE)(1).

Diatom oxylipins have been observed to deleteriously impact copepod reproductive success. However, field studies have revealed very variable and case-dependent results. Therefore, the plasticity of diatom oxylipin metabolism was studied among four clones of the marine diatom Skeletonema marinoi Sarno et Zingone. Diatom oxylipin metabolism was studied by two lipoxygenase (LOX) activity assays carried out at different pH values and by oxylipin quantification. The four clones showed no major metabolic differences in terms of protein content or growth rate. However, two of the clones produced significantly higher levels of oxylipins than the other two. LOX activity measurements also indicated clonal variability in fatty acid oxidative metabolism. The presence of clone-specific differences in oxylipin metabolism may play a role in shaping diatom population dynamics by conferring selective advantages to certain clones.

Dynamics of diatom viruses on the western coast of Japan

Diatoms are prominent primary producers that play an important role in global carbon cycles. Diatom dynamics are, therefore, important for biogeochemistry, fisheries and earth science. Viral infection is now assumed to be one of the most significant factors affecting diatom dynamics. However, few studies, based on practical field data, have focused on the relationship between diatoms and viruses in natural waters. To elucidate on this relationship, we assessed the temporal change in the abundance of diatoms and their viruses on the western coast of Japan from 2004 to 2009. Three species of diatoms were used as hosts to enumerate viruses in water and sediment sam- ples: Chaetoceros debilis, C. tenuissimus and C. salsugineum. Diatom viruses in the water column rapidly increased during C. debilis and C. tenuissimus blooms, maintaining a high abundance throughout the blooming period. These data suggest the potential importance of viruses in control- ling the population dynamics of diatoms in natural environments. The number of viruses in sediments fluctuated considerably, even during non-blooming periods, which suggests that virus-infected diatom cells were supplied to the sediments from water-column populations throughout the year. This process might be an important strategy to reduce the impact of viruses on diatom populations. Because diatom populations are thought to have an intimate relationship with their viruses in natural waters, further studies of diatom dynamics should include the effects of viral infections.

Copepod grazing during an iron-induced diatom bloom in the Antarctic Circumpolar Current (EisenEx): I. Feeding patterns and grazing impact on prey populations

Feeding activity, selective grazing and the potential grazing impact of two dominant grazers of the Polar Frontal Zone, Calanus simillimus and Rhincalanus gigas, and of copepods < 2 mm were investigated with incubation experiments in the course of an iron fertilized diatom bloom in November 2000. All grazers were already actively feeding in the low chlorophyll waters prior to the onset of the bloom. C. simillimus maintained constant clearance rates and fed predominantly on diatoms. R. gigas and the small copepods strongly increased clearance and ingestion of diatoms in response to their enhanced availability. All grazers preyed on microzooplankton, most steadily on ciliates, confirming the view that pure herbivory appears to be the exception rather than the rule in copepod feeding. The grazers exhibited differences in feeding behavior based on selectivity indices. C. simillimus and R. gigas showed prey switching from dinoflagellates to diatoms in response to the phytoplankton bloom. All grazers most efficiently grazed on large diatoms leading to differences in daily losses for large and small species, e.g. Corethron sp. or Thalassionema nitzschioides. Species-specific diatom mortality rates due to grazing suggest that the high feeding activity of C. simillimus prior to and during the bloom played a role in shaping diatom population dynamics.

Phytoplankton abundance, community structure and nutrients in cultural areas of Daya Bay, South China Sea

In order to provide a better understanding of phytoplankton structure and their relationship with environmental parameters in typical maricultural areas in Southern China, observations were carried out to estimate the phytoplankton structure and nutrients in eight stations in annual survey from July 1997 to June 1998 and three springs from 1998 to 2000 in Daya Bay, South China Sea. Phytoplankton and ambient chemical, physical, meteorologic data were examined. A total of 198 species of phytoplankton were identified. Diatom was the most diversified group in which 98 species in 39 genera were observed, while dinoflagellate was the second one with 83 species in 14 genera. The annual mean chlorophyll a concentration and cell density was 4.7 mg/m 3 and 425 cells/ml, respectively. The abundance of total phytoplankton ranged from 10 cells/ml to 6698 cells/ml. Water temperatures were mostly over 20 °C with the annual average of 25.0 °C. Salinities remained constant except for the rainy typhoon season from July to September. DSi was sufficient in Daya Bay, and never limited for the growth of diatoms. Diatoms were preponderant in all seasons, while dinoflagellates were prevalent only in spring. Dominant diatoms included Asterionella japonica, Chaetoceros spp., Pseudo-nitzschia spp., Skeletonama costatum and Thalassiosira subtilis, which predominated alternately or co-dominated together. An unidentified dinoflagellate, Gymnidinium sp., was the dominant dinoflagellate, causing two blooms in May 1998. Results from statistical analyses revealed that DIN and DSi had high loadings in population dynamics of diatoms, and DIP was important for dinoflagellates. Meanwhile, peak abundances of diatoms were coinstantaneous with the low values of DIN and DSi concentrations, while high abundances of dinoflagellates with low levels of DIP. The variety in nutrient requirement and utilization by diatoms and dinoflagellates resulted in frequent occurrence of spring blooms caused by them. From the results of this survey, it could be suggested that appropriate water temperature, salinity, sufficient DSi, as well as quick recovery of nutrients, played important roles in the high abundance of phytoplankton and frequent outbreak of blooms in Daya Bay.

Previously Unknown Virus Infects Marine Diatom

Diatoms are a major phytoplankton group that play important roles in maintaining oxygen levels in the atmosphere and sustaining the primary nutritional production of the aquatic environment. Among diatoms, the genus Chaetoceros is one of the most abundant and widespread. Temperature, climate, salinity, nutrients, and predators were regarded as important factors controlling the abundance and population dynamics of diatoms. Here we show that a viral infection can occur in the genus Chaetoceros and should therefore be considered as a potential mortality source. Chaetoceros salsugineum nuclear inclusion virus (CsNIV) is a 38-nm icosahedral virus that replicates within the nucleus of C. salsugineum. The latent period was estimated to be between 12 and 24 h, with a burst size of 325 infectious units per host cell. CsNIV has a genome structure unlike that of other viruses that have been described. It consists of a single molecule of covalently closed circular single-stranded DNA (ssDNA; 6,005 nucleotides), as well as a segment of linear ssDNA (997 nucleotides). The linear segment is complementary to a portion of the closed circle creating a partially double-stranded genome. Sequence analysis reveals a low but significant similarity to the replicase of circoviruses that have a covalently closed circular ssDNA genome. This new host-virus system will be useful for investigating the ecological relationships between bloom-forming diatoms and other viruses in the marine system. Our study supports the view that, given the diversity and abundance of plankton, the ocean is a treasury of undiscovered viruses.

GROWTH INHIBITION AND TOXICITY OF THE DIATOM ALDEHYDE 2‐TRANS, 4‐TRANS‐DECADIENAL ON THALASSIOSIRA WEISSFLOGII (BACILLARIOPHYCEAE)1

A common aldehyde present in marine and freshwater diatoms, 2‐trans, 4‐trans‐decadienal (A3), is involved in the wound‐activated response of diatoms to copepod grazing. Upon breakage of the diatom cell membrane, aldehydes are enzymatically produced by the rapid conversion of precursors and strongly impact copepod reproduction by impairing egg production and hatching success, inducing teratogenic embryos modifications. In this study, A3 was assayed with the marine diatom Thalassiosira weissflogii (Grunow) Fryxell et Hasle. The aldehyde concentration necessary to reduce 50% growth rate (EC50) was 0.29 mg·L−1. Decadienal was found to inhibit T. weissflogii cell growth in a dose‐ and time‐dependent manner, with irreversible effects after 24 h of exposure. Decadienal induced a degenerative process, through modifications of cell membrane characteristics, interference with cell cycle progression, and with cell metabolic activity, leading to cell death. A preferential action of A3 on dividing cells was observed. Photosynthetic efficiency significantly decreased upon exposure to the aldehyde, paralleled by an increase in diatoxanthin, suggesting a protective role of this xanthophyll, usually involved in photoprotection. Dying cells exhibited the morphological and biochemical features that bear close resemblance to apoptosis of mammalian cells, including cell shrinkage, chromatin condensation, and degradation of nuclear DNA to nucleosomal size fragments. These data are the first direct evidence to show aldehydes are toxic to diatoms. We suggest a possible nontoxic role of such compounds as chemical signals of unfavorable conditions within the phytoplankton communities, which may be relevant for the population dynamics of diatoms during blooms.

Feeding by the heterotrophic dinoflagellate Protoperidinium bipes on the diatom Skeletonema costatum

The heterotrophic dinoflagellate Protoperidinium bipes is a predominant heterotrophic dinoflagellate (maximum density = 137 cells ml -1 ) during diatom blooms. To investigate its role as a grazer in the population dynamics of diatoms, we measured growth and ingestion rates of P. bipes when feeding on the common diatom Skeletonema costatum. We also calculated grazing coefficients by combining field data on abundances of P. bipes and co-occurring S. costatum with laboratory data on ingestion rates obtained in the present study. Specific growth rates of P. bipes increased continuously with increasing concentration of S. costatum. The maximum specific growth rate of P. bipes on S. costa- tum was 1.37 d -1 when data for the growth rate were fitted to a Michaelis-Menten equation. The thresh- old prey concentration (where net growth = 0) was 111 ng C ml -1 (4270 cells ml -1 ). Maximum ingestion and clearance rates of P. bipes on this diatom were 2.9 ng C grazer -1 d -1 (112 cells grazer -1 d -1 ) and 1.0 µl grazer -1 h -1 , respectively. P. bipes exhibited the highest maximum swimming speed (ca. 8.3 mm s -1 ) and maximum volume-specific clearance rate (5.4 × 10 6 h -1 ) among Protoperidinium species so far reported. Calculated grazing coefficients by P. bipes on S. costatum (0.001 to 0.034 h -1 , i.e. 0.1 to 3.4% of S. costa- tum populations were removed by a P. bipes population in 1 h) were much higher than those by co- occurring Acartia spp. (<0.002 h -1 ). The results of the present study suggest that P. bipes sometimes has

Rapid evolution of a sexual reproduction gene in centric diatoms of the genus Thalassiosira.

Sexual reproduction is commonly assumed to occur in the vast majority of diatoms due to the intimate association of this process with cell size control. Surprisingly, however, little is known about the impact of sexual events on diatom population dynamics. The Sig1 gene is strongly upregulated during sexual reproduction in the centric diatom Thalassiosira weissflogii and has been hypothesized to encode a protein involved in gamete recognition. In the present study, degenerate PCR primers were designed and used to amplify a portion of Sig1 from three closely related species in the cosmopolitan genus Thalassiosira, Thalassiosira oceanica, Thalassiosira guillardii, and Thalassiosira pseudonana. Identification of Sig1 in these three additional species facilitated development of this gene as a molecular marker for diatom sexual events. Examination of the new sequences indicated that multiple copies of Sig1 are probably present in the genome. Moreover, compared to the housekeeping gene beta-tubulin, the Sig1 genes of isolates of T. weissflogii collected from different regions of the Atlantic and Pacific oceans displayed high levels of divergence. The Sig1 genes of the four closely related Thalassiosira species also displayed high levels of sequence divergence compared to the levels observed with a second gene, Fcp, probably explaining why Sig1 could not be amplified from more distantly related species. The high levels of sequence divergence both within and between species suggest that Sig1 is rapidly evolving in a manner reminiscent of the manner observed in other genes that encode gamete recognition proteins. A simple model is presented for Sig1 evolution and the implications of such a rapidly evolving sexual reproduction gene for diatom speciation and population dynamics.

Size reduction, reproductive strategy and the life cycle of a centric diatom

The life cycle of Aulacoseira subarctica (O. Müller) Haworth in Lough Neagh, Northern Ireland, is described. Cell numbers can reach up to 17000 per millilitre in spring. Most cells sediment to the bottom after silica limitation and go into a resting state during summer. The inoculum in autumn partly comes from resuspension, with the surviving cells (0.5-5%) continuing to grow through the winter, doubling every one to two weeks. T he population goes through a size reduction and regeneration cycle linked to sexual reproduction. Gametes are only produced in narrower cells (3.8-7.4 um diameter), usually after interruptions in growth caused by low light conditions (surface irradiance 100-150 pE m -2 s-1), but availability of nutrients, especially silica and nitrogen, is also important. Even the highest densities of auxospores (20 m1 -1) represent only a small proportion of the total cells present (0.16%). Size regeneration results in initial cells with diameters (14.8 ± 2 pm) about three times those of the parent. Larger parent cells usually give rise to larger initial cells. Subsequently, cell division leads to a decrease in population diameter, because of the way new valves are laid down below the girdle bands. Reductions are largest in broader cells (0.32 um per division) and gradually decrease as cells get narrower. Occasionally large reductions, up to 1 um, follow periods of environmental stress. By combining these results with studies of changes in cell size (width, length and volume) in related individuals along filaments, it was possible to explain why there have been difficulties in applying the MacDonald-Pfitzer hypothesis to natural populations. Theoretically, the life cycle in L. Neagh might extend over 100 divisions or 15 years but, in practice, cells reach a sexually inducible size in 4-6 years. The discrepancy is because environmental factors (e.g. sedimentation, resuspension, parasitism, etc.) are also important in size selectivity. The interaction of these factors, when combined with intermittent sexual reproduction at low frequencies, results in a relatively stable population size distribution, where there are always some cells in the size range in which sexual differentiation can be induced. Overall, the results demonstrate, that for a full understanding of diatom population dynamics, it is important to quantify events over complete life cycles.